Turbomolecular pumps are often employed as a component of the vacuum system used to evacuate devices such as scanning electron microscopes (SEMs) and lithography devices.
It is common for said turbomolecular pumps to comprise an oil free, passive permanent magnetic bearing arrangement, located in the high vacuum end of the pump, to provide a substantially friction free, dry bearing arrangement free of lubricating materials that might otherwise cause contamination in the evacuated volume.
As described in EP2705263, known arrangements of passive permanent magnetic bearings employ a plurality of individual axially stacked ring magnets. Examples of such arrangements are shown in FIGS. 1 and 2.
FIG. 1 illustrates a section of a typical turbomolecular pump 200 comprising a series of rotor blades 106 extending outwardly from a rotor shaft 108. A passive magnetic bearing arrangement 100, 110 is located at the high vacuum (inlet) end of the shaft 108. The bearing arrangement 100, 110 comprises a series of three individual permanent magnet rings 100 fixed to the pump housing surrounded concentrically by a series of three individual permanent magnet rings 110 which are fixed to, and rotate with, the rotor arrangement 106, 108 about the axis 102.
A cross section of a further example of a passive permanent magnetic bearing arrangement 10 for a turbomolecular pump (not shown) is illustrated in more detail in FIG. 2. In this example the bearing arrangement 10 comprises an array 12 of four outer rotating permanent magnet rings 12a, 12b, 12c and 12d and an array 14 of four inner non-rotating permanent magnetic rings 14a, 14b, 14c and 14d arranged such that the outer, rotating, array 12 surrounds the inner, static, array 14 in a concentric manner. The magnets are all formed of rare earth magnetic material, such as samarium-cobalt. The outer array 12 is attached to the rotor of a turbomolecular pump (not shown) with the static array 14 attached to the stator of said pump. For reasons of mechanical strength and practical construction, it is normal for the outer array of rings to form the rotating part of the bearing arrangement and the inner rings to form the stationary part.
In this example the magnetisation, (that is, the polarization), of the magnetic rings 12a to 12d and 14a to 14d in each array 12, 14 respectively is substantially aligned with the axis of rotation 102 of the pump rotor (not shown). The direction of magnetisation (polarization) has been indicated by the arrows, with the head of each arrow indicating the north pole.
The magnets are arranged within each array such that they are in mutual repulsion with each other; that is proximate magnets in an array meet their nearest neighbouring magnet in the same array with the same pole (e.g. magnets 12a and 12b meet each other with their south poles). The outer magnetic rings 12a, 12d, 14a, 14d in each array have their north poles facing outermost.
The magnets 12a to 12d and 14a to 14d in each array 12, 14 of the arrangement 10 are orientated to provide a mutual repulsion between the arrays 12, 14 and therefore create an almost frictionless bearing.
A great many other configurations are possible, using different numbers of rings, with axial or radial magnetisation, and arranged for either repulsive or attractive forces between rotor and stator. Although a variety of configurations are possible, they all perform optimally when the direction of magnetisation in the rings is perfectly symmetrical with respect to their rotational axis 102.
The magnetisation in the rings 12a to 12d of the rotating array 12 is shown in FIG. 2 as perfectly symmetrical with respect to their geometric (rotational) axis 102. However, in reality, the axial magnetisation of each magnetic ring 12a to 12d (and, similarly, for magnets 14a to 14d) is imperfect due to the practical limitations of their manufacturing process.
Although the production of magnets is well known to those skilled in the art, in order to illustrate how the imperfections in the process arise and cause problems in turbomolecular pumps, a simplified version will be described herein.
The most common method of producing magnets is via powder metallurgy. The process starts by forming a fine powder which is then compacted and sintered together, before being charged, or magnetized.
The fine powder, which is formed by several steps, is provided with a specific particle size to contain material with one preferred magnetic orientation.
Following the formation of the powder it is compacted to the desired shape. The two well-known techniques used for this process are axial/transverse pressing and isostatic pressing. Both methods essentially involve aligning and fixing the particles so all the magnetic regions in the finished magnet are pointing in a single direction.
In axial/transverse pressing, the powder is placed into a rigid cavity shaped to match the shape of the final magnet and then and compressed with a pressing tool. Before the compression occurs, an aligning magnetic field is applied to the powder to ensure that all the particles are aligned in the same direction. The act of compression fixes, or “freezes-in” this alignment.
Isostatic pressing is where a flexible container is filled with the powder, the container is then sealed, and an aligning field applied. The container is then isostatically pressed using a hydraulic fluid (e.g. water), thus, pressure is applied to the outside of the sealed container, compacting it equally on all sides. By isostatic pressing it is both possible to make large magnets and, because the compacting pressure is applied equally on all sides ensuring the powder remains in relatively good alignment, with relatively high magnetic energy.
The pressed parts are then sintered in a vacuum sintering furnace, with the temperature and atmosphere around the magnet being specified dependent on the type and grade of magnet being produced. Rare earth materials are heated to a sintering temperature and allowed to densify over time. The SmCo magnets used in the above examples have the additional requirement of a solutionising heat treatment after sintering.
When the sintering process is complete the magnets have rough surfaces and only approximate dimensions so require further treatment by, for example, grinding of the internal and external surfaces to produce the final finish. At this point they still exhibit no external magnetic field.
Following the finishing process, the magnet then requires magnetizing to produce an external magnetic field. This can be accomplished in a solenoid comprising a hollow cylinder into which various magnet sizes and shapes can be placed, or with other devices designed to impart unique magnetic patterns.
Thus when each individual magnet in the array is made, they can each pick up minor variations in the orientation of the direction of magnetic field. Therefore each individual ring magnet has slight imperfections with respect to each other and so the axial alignment of each of the polarizations in an array with respect to the rotational axis 102 will also be imperfect (asymmetric) with respect to each other.
This is illustrated in FIGS. 3a and 3b. The largest magnetic asymmetry observed in axially magnetised permanent magnetic rings is usually a small angular error such that the magnet's axis is displaced from the rotational axis 102 by an angle of a few degrees as indicated in FIG. 3a. Depending on the quality, or grade, of the magnet the angular error, θ, can be as much as 3°. This error may be regarded as a small perturbation from the ideal axial magnetisation; in effect a transverse magnetic dipole moment 8 superimposed on the intended axial dipole moment 6 as illustrated in FIG. 3b. 
In addition to the transverse dipole (first order) asymmetry, higher order asymmetries exist, for example quadrupole and hexapole asymmetries. The magnitude, or magnetic field strength, of the asymmetry usually decreases as the number of poles increases.
Where these small asymmetries occur in any of the rings 12a to 12d of the rotating magnet array 12, a time varying magnetic field is generated (the magnetic field is constant for the static magnets 14a to 14d). These 2, 4 and 6 pole asymmetries generate time varying magnetic fields at frequencies of 1, 2, 3 times the rotational speed of the pump rotor respectively.
The performance of scanning electron microscopes is highly susceptible to mechanical vibrations or stray magnetic fields emitted from turbomolecular pumps. The stray fields are known to directly interfere with the electron beam or with the instruments' electrical circuits.
One known way to overcome the above described problems of stray first and second order magnetic fields, as described in EP2705263, is by assembling the rotating magnet array for a permanent magnet bearing arrangement by a method which effectively cancels out the stray fields of each individual magnet. This is achieved by first measuring/characterising the size and phase (vectors) of at least the first and second order transverse stray magnetic fields of a plurality of magnets, namely the transverse dipole and quadrupole stray fields. Then, for at least four ring magnets individually in relation to a reference point on said ring magnets, calculating the relative angular orientation and relative magnetic polarity direction of each of said at least 4 magnets within the array such that, when the array is assembled, it will provide the minimum time-varying magnetic field. This is the optimum relative orientation of the magnets at which all of the stray fields for each of the magnets are substantially cancelled out.
However, one problem with the above described method is that many magnets have to be characterised in order to find acceptable combinations of magnets for a pump. In other words, in order to find a combination of four ring magnets for which the stray magnetic fields can be substantially cancelled out the initial characterisation of more than four magnets is needed.
It is the intention of the present invention to overcome the above mentioned problems